Image reading apparatus and image reading method

ABSTRACT

An image reading apparatus having a color line sensor and a monochromatic line sensor starts to read on the basis of the color reading start position when reading the image of a document in color and starts to read on the basis of the monochromatic reading start position when reading the image of a document in monochrome.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is based upon and claims the benefit of priority fromprior Japanese Patent Application No. 2003-056324, filed Mar. 3, 2003,the entire contents of which are incorporated herein by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to an image reading apparatus and an imagereading method which scan the reading face of a document optically andconvert the scanned data into color image data or monochromatic imagedata.

2. Description of the Related Art

In a conventional image reading apparatus (or a color image readingapparatus) which reads images on a document in color, the entiredocument is scanned optically by moving the carriage and the colorimages are read by the color CCD sensor. The conventional color imagereading apparatus uses a 3-line CCD sensor as a color CCD sensor. The3-line CCD sensor is composed of the following three line CCD sensors: afirst line CCD sensor that outputs a red component (R signal), a secondline CCD sensor that outputs a green component (G signal), and a thirdline CCD sensor that outputs a blue component (B signal). In an imagereading apparatus with three line CCD sensors, when a monochromaticimage is read, a monochromatic image is created on the basis of thesignals (RGB signals) outputted from the three line sensors.

In the conventional image reading apparatus, the output of each of thethree line CCD sensors constituting the 3-line CCD sensor is not alwaysuniform. This nonuniformity results from variations in the sensitivityof a pixel unit in each line CCD sensor and a decrease in the amount oflight around the lens caused by the light distribution characteristic ofthe exposure lamp that illuminates the document and the characteristicof the lens. Thus, in the conventional image reading apparatus, areference plate (shading correction plate) acting as a white referenceis read by the 3-line CCD sensor. On the basis of the result of thereading, the output signal from the 3-line CCD sensor is corrected (orthe shading is corrected).

Therefore, in the image reading apparatus with the 3-line CCD sensor,the white reference plate is always read in the same reading position.This is because the image reading apparatus with the 3-line CCD sensorreads not only color images but also monochromatic images with the threeline CCD sensors. Consequently, the conventional image reading apparatuswith the three-line CCD sensor reads the shading correction plate in thefixed reading position, regardless of whether the image of the documentis read in color or in monochrome, and makes a shading correction of theoutput signal from each line sensor.

BRIEF SUMMARY OF THE INVENTION

It is an object of the present invention to provide an image readingapparatus and an image reading method which are capable of not onlyreading color images and monochromatic images efficiently but alsomaking a shading correction effective and the image reading apparatusmore compact and stable by making the width of the reference platesmaller.

According to an aspect of the present invention, there is provided animage reading apparatus which reads images on the reading face of adocument in color or in monochrome, the image reading apparatuscomprising: a line sensor which includes a color line sensor for readinga color image and a monochromatic line sensor for reading amonochromatic image different from the color line sensor; a scanningsection on which an optical system for directing light from the readingface of the document to the line sensor is provided; a driving mechanismwhich moves the scanning section in a feed direction with respect to thereading face of the document; a reference plate which is provided infront of the leading edge of the reading face of the document in thefeed direction in which the scanning section is moved by the drivingmechanism; and a control section which, when the reading face of thedocument is read in color, starts to read the reference plate with thecolor line sensor at the time that the scanning section moved in thefeed direction by the driving mechanism reaches a color reading startposition for the reference plate, thereby reading the reference platewith the color line sensor, and which, when the reading face of thedocument is read in monochrome, starts to read the reference plate withthe monochromatic line sensor at the time that the scanning sectionmoved in the feed direction by the driving mechanism reaches amonochromatic reading start position for the reference plate, therebyreading the reference plate with the monochromatic line sensor.

According to another aspect of the present invention, there is providean image reading method used in an image reading apparatus whichcomprises a line sensor which includes a color line sensor for reading acolor image and a monochromatic line sensor for reading a monochromaticimage different from the color line sensor, a scanning section on whichan optical system for directing light from the reading face of thedocument to the line sensor is provided, a driving mechanism which movesthe scanning section in a feed direction with respect to the readingface of the document, and a reference plate which is provided in frontof the leading edge of the reading face of the document in the feeddirection in which the scanning section is moved by the drivingmechanism, the image reading method comprising: when the reading face ofthe document is read in color, starting to read the reference plate withthe color line sensor at the time that the scanning section moved in thefeed direction by the driving mechanism reaches a color reading startposition for the reference plate and thereby reading the reference platewith the color line sensor; and when the reading face of the document isread in monochrome, starting to read the reference plate with themonochromatic line sensor at the time that the scanning section moved inthe feed direction by the driving mechanism reaches a monochromaticreading start position for the reference plate and thereby reading thereference plate with the monochromatic line sensor.

Additional objects and advantages of the invention will be set forth inthe description which follows, and in part will be obvious from thedescription, or may be learned by practice of the invention. The objectsand advantages of the invention may be realized and obtained by means ofthe instrumentalities and combinations particularly pointed outhereinafter.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING

The accompanying drawings, which are incorporated in and constitute apart of the specification, illustrate an embodiment of the invention,and together with the general description given above and the detaileddescription of the embodiment given below, serve to explain theprinciples of the invention.

FIG. 1 shows the configuration of a 4-line CCD sensor provided in animage reading apparatus according to an embodiment of the presentinvention;

FIG. 2 schematically shows the configuration of the image readingapparatus according to the embodiment;

FIG. 3 is a block diagram of the control system of the image readingapparatus;

FIG. 4 is a flowchart to help explain the operation of the entire imagereading apparatus roughly;

FIG. 5 is a diagram to help explain the operation in a case where thefour line sensors read a shading correction plate in turn;

FIG. 6 is a diagram to help explain a shading correction plate readingoperation in a first reading method;

FIG. 7 is a diagram to help explain a shading correction plate readingoperation in a second reading method;

FIG. 8 is a diagram to help explain a shading correction plate readingoperation in a third reading method;

FIG. 9 is a diagram to help explain a shading correction plate readingoperation in a fourth reading method;

FIG. 10 is a diagram to help explain a shading correction plate readingoperation in a fifth reading method; and

FIG. 11 is a diagram to help explain a shading correction plate readingoperation in a sixth reading method.

DETAILED DESCRIPTION OF THE INVENTION

Hereinafter, referring to the accompanying drawings, an embodiment ofthe present invention will be explained.

FIG. 1 shows a configuration diagram of a 4-line CCD sensor 1 providedin an image reading apparatus according to an embodiment of the presentinvention.

As shown in FIG. 1, the 4-line CCD sensor 1 comprises a red line sensorR that converts the red component of the incident lightphotoelectrically into an R signal representing the intensity of red, agreen line sensor G that converts the green component of the incidentlight photoelectrically into a G signal representing the intensity ofgreen, a blue line sensor B that converts the blue component of theincident light photoelectrically into a B signal representing theintensity of blue, and a black-and-white line sensor BW that convertsthe black and white components of the incident light photoelectricallyinto a BW signal representing the intensity of black and white.

In the 4-line CCD sensor 1, the red line sensor R, green line sensor G,and blue line sensor B constitute a color line sensor that reads imagesin color. The black-and-white line sensor BW constitutes a monochromaticline sensor that reads images in monochrome.

The red line sensor R is composed of a CCD line sensor with a redfilter. Therefore, the red line sensor R can take in only the redcomponent of the incident light and output an R signal.

The green line sensor G is composed of a CCD line sensor with a greenfilter. Therefore, the green line sensor G can take in only the greencomponent of the incident light and output and a G signal.

The blue line sensor B is composed of a CCD line sensor with a bluefilter. Therefore, the blue line sensor B can take in only the bluecomponent of the incident light and output and a B signal.

In the 4-line CCD sensor 1, the line sensors R, G, B, BW are arranged inparallel at specific intervals. In an example of FIG. 1, the linesensors are arranged in this order: R, G, B, BW. Each of the spacingbetween the red line sensor R and the green line sensor G and thespacing between the green line sensor G and the blue line sensor B isequivalent to eight lines. The spacing between the blue line sensor Band the black-and-white line sensor BW is equivalent to 12 lines.

Specifically, as the color line sensors, the red line sensor R, greenline sensor G, and blue line sensor B are arranged in parallel atintervals of eight lines. As the monochromatic line sensor, theblack-and-white sensor BW is arranged in parallel with the blue linesensor B acting as a color line sensor, spaced 12 lines apart. One linecontains, for example, 7450 pixels. One pixel has a size of 4.7 μm×4.7μm.

Next, the configuration of the image reading apparatus provided with the4-line CCD sensor will be explained.

FIG. 2 shows a configuration of the image reading apparatus according tothe embodiment of the present invention.

As shown in FIG. 2, the image reading apparatus 10 comprises a shadingcorrection plate 11, a document table 12, an exposure lamp 14, areflector 15, a first mirror 16, a first carriage 18, a second carriage20, a second mirror 22, a third mirror 24, an image forming lens 26, adriving motor 30, and a control unit (control board) 32.

The shading correction plate 11 is a reference plate for making ashading correction of the output signal of each line sensor of the4-line CCD sensor on a pixel basis. The shading correction plate 11shows a white reference. Specially, in the shading correction, theoutput data in pixels from each line sensor of the 4-line CCD sensor 1is corrected on the basis of the data read from the shading correctionplate 11.

The width in the feed direction of the shading correction plate 11 isdetermined by the spacing between the individual line sensors, thenumber of reads (the number of reading lines), the reading magnification(the travel speed of the first carriage), and others. In theconfiguration of FIG. 2, the shading correction plate 11 is providedjust in front of the document table 12 in the direction in which animage is read with the first carriage 18 (or in the feed direction).

The shading correction plate 11 has to be read before the document imageis read. It also has to be read in a state where the first carriage 18is moving stably. For this reason, the shading correction plate 11 hasto be provided in front of the document image reading area (or in frontof the leading edge of the document) and behind the position where thetravel speed of the first carriage 18 started from a specific standbyposition becomes stable.

Since the above restriction is put on the installation location of theshading correction plate 11 in the image reading apparatus 10, it isdesirable that the width in the feed direction of the shading correctionplate 11 should be as narrow as possible. In addition, it is desirablethat the shading correction plate 11 should be read efficiently.

The document table 12 is composed of a colorless, transparent member,such as glass, which permits light to pass through. The document table12 is provided with a document cover (not shown). The document on thedocument table 12 is pressed against the glass surface of the documenttable 12 by the document cover (not shown).

The exposure lamp 14 functions as a light source to eliminate theshading correction plate 11 and the document D put on the document table12. The reflector 15 reflects a part of the light from the exposure lamp14 and illuminates the document D. The first mirror 16 deflects thereflected light from the shading correction plate 11 or the document Din a specific direction.

The first carriage 18 is provided with the exposure lamp 14, reflector15, first mirror 16, and others. The first carriage 18 is provided underthe document table 12 so as to be movable in parallel with the documenttable 12. The first carriage 18 is moved back and forth under thedocument table 12 by a driving motor 30 connected to the carriage 18 viaa toothed belt (not shown) and others. The driving motor 30 is composedof a stepping motor driven by a driving pulse signal or the like fromthe control unit 32.

Furthermore, under the document table 12, the second carriage 20 isprovided so as to be movable in parallel with the document table 12. Onthe second carriage 20, the second mirror 22 and third mirror 24 whichdeflect sequentially the reflected light from the document D deflectedby the first mirror 16 are so provided that they cross each other atright angles. The driving force of the driving motor 30 is imparted tothe second carriage 20 by the toothed belt and others which drive thefirst carriage 18, with the result that the second carriage 20 is movedaccording to the movement of the first carriage 18. The second carriage20 is moved in parallel with the document table 12 at a speed half ofthe speed of the first carriage 18.

In addition, under the document table 12, there are provided the imageforming lens 26 which converges the reflected light from the thirdmirror 24 mounted on the second carriage 20 and the 4-line CCD sensor 1which receives the reflected light converged by the image forming lens26 and converts it photo-electrically. The image forming lens 26 isprovided in a movable manner via a driving mechanism (not shown) in aplane that includes the optical axis of the light deflected by the thirdmirror 24. The image forming lens 26 itself moves, thereby forming animage from the reflected light at a desired magnification. Then, in the4-line CCD sensor 1, the individual line sensors R, G, B, BW convertphotoelectrically the light coming in via the image forming lens 26pixel by pixel and output the resulting signals to the control unit 32.

Next, the configuration of the control system of the image readingapparatus 10 will be explained.

FIG. 3 is a schematic block diagram of the control system of the imagereading apparatus 10.

On the control board 32 of the image reading apparatus 10, there areprovided a CPU 40, a ROM 41, a RAM 42, a signal processing section 43,and a driving control section 44. The CPU 40 controls the entire imagereading apparatus 40. The ROM 41 is a memory in which control programsfor performing the image reading operation and others are stored. TheRAM 42 is a memory which stores data temporarily. The signal processingsection 43 processes the signal from the 4-line CCD sensor 1 and outputsthe resulting signal to the outside world. The driving control section44 has a motor driver which drives the driving motor 30.

The signal processing section 43 has a pre-processing circuit 51, ashading correction circuit 52, a line-to-line correction circuit 53, andan image processing circuit 54.

The preprocessing circuit 51 carries out processes, including an A/Dconversion process of converting the analog signal from the 4-line CCDsensor into a digital signal.

The shading correction circuit 52 makes correction in pixels on thebasis of the result of the reading of the shading correction plate 11 bythe 4-line CCD sensor 1. Specifically, the shading correction circuit 52creates correction data for each pixel on the basis of the result of thereading of the shading correction plate 11 as a white reference plate bythe 4-line CCD sensor 1. Furthermore, the shading correction circuit 52corrects the output signals from the individual line sensors R, G, B, BWof the 4-line CCD sensor 1 according to the correction data created onthe basis of the result of the reading of the shading correction plate11.

For example, on the basis of the black reference data previously read bythe line sensors R, G, B, BW (the output signals from the respectiveline sensors when the exposure lamp 14 is off) and the white referencedata read from the shading correction plate 11 by the line sensors R, G,B, BW (the output signals from the respective line sensors when theshading correction plate 11 as a white reference plate is read) theshading correction circuit 52 corrects the output signals from the linesensors R, G, B, BW in pixels in reading the document image by using thefollowing equation:I=K×(S−B)/(W−B)

where I is the signal after correction, K is a coefficient, S is theoutput signal before correction (the output signal from the linesensor), B is the black reference data, and W is the white referencedata.

The line-to-line correction circuit 53 aligns the data from the red linesensor R, the data from the green line sensor G, and the data from theblue line sensor B with one another. Specifically, the red, green, andblue line sensors are so arranged that their lines are shifted severalpixels from one another. For this reason, to create a color image, thephases of the data from the line sensors R, G, and B have to be alignedwith one another according to the movement speed in the feed direction.

For example, in the configuration of FIG. 1, the red, green, and blueline sensors R, G, and B as color line sensors are arranged in thisorder: R, G, B, or in the order of scanning. The red line sensor R andthe green line sensor G are so arranged that they are spaced eightpixels apart. The green line sensor G and the blue line sensor B are soarranged that they are spaced eight pixels apart. In this case, if themagnification ratio is 25% to 400%, a positional correction of 2 to 32lines has to be made between R and G and between G and B in the datafrom the line sensors R, G, B.

For example, when the blue line sensor B is used as a reference, theline-to-line correction circuit 53 makes a positional adjustment of 4 to64 lines to the data from the red line sensor R and a positionaladjustment of 2 to 32 to the data from the green line sensor G. Makingsuch positional adjustments, the line-to-line correction circuit 53superimposes the data of the R signal, G signal, and B signal andcreates a color image with no off-shade part.

The image processing circuit 54 carries out an image process and outputsthe resulting image to the outside world. For example, when outputting acolor image, the image processing circuit 54 makes a color correction ofthe data subjected to the line-to-line correction and outputs theresulting data to the outside world. When outputting a monochromaticimage, the image processing circuit 54 effects the filtering of the dataof the BW signal passed through the line-to-line correction circuit andoutputs the resulting data to the outside world.

To the CPU 40, an operation section 60 is connected. The user'soperation instruction is inputted to the operation section 60. Theoperation section 60 is provided with, for example, a setting key forsetting a read magnification, an image select key for selecting eithercolor or monochrome, a specify key for specifying the start of reading.For example, when the user specifies the reading mode for the documentfrom the operation section 60 and presses the key for specifying thestart of reading, the CPU 40 starts to read the document image accordingto the specified reading mode.

Furthermore, to the CPU 40, a switching circuit 61 and a switchingcircuit 62 are connected. The switching circuit 61 is a circuit whichswitches between the G signal from the green line sensor G and the BWsignal from the black-and-white line sensor BW among the signalssupplied from the 4-line CCD sensor 1 to the signal processing section43. The switching circuit 62 is a circuit which switches between the Bsignal from the blue sensor B and the BW signal from the black-and-whiteline sensor BW among the signals supplied from the 4-line CCD sensor 1to the signal processing section 43.

Specifically, in the reading mode of reading images in color (colorreading mode), the CPU 40 makes the G signal effective with theswitching circuit 61 and the B signal effective with the switchingcircuit 62. In this case, the 4-line CCD sensor 1 supplies the R signalfrom the red line sensor R, the G signal from the green line sensor G,and the B signal from the blue line sensor B to the signal processingsection 43. This enables the 4-line CCD sensor 1 to read a color image.

Furthermore, in the reading mode of reading images in monochrome(monochromatic reading mode), the CPU 40 makes not only the BW signaleffective with the switching circuit 61 but also the BW signal effectivewith the switching circuit 62. In this case, the 4-line CCD sensor 1supplies the BW signal from the black-and-white line sensor BW to thesignal processing section 43. This enables the 4-line CCD sensor 1 toread a monochromatic image.

With the configuration of FIG. 3, when reading a monochromatic image,the 4-line CCD sensor 1 supplies the BW signals of the two channels tothe signal processing section 43. In this case, one channel supplies theBW signal for an even number of lines and the other supplies the BWsignals for an odd number of lines.

Next, the operation of the image reading apparatus 1 will be explainedbriefly.

FIG. 4 is a flowchart to help explain the read operation of the imagereading apparatus 10.

When the power key (not shown) is pressed, the CPU 40 executes aninitialize operation (step S11). In the initialize operation, the CPU 40carries out the initial operation of the entire image reading apparatus10, including the initialization of the driving reference point and thesetting of the amplification factor by the signal amplifying section.

After the initialize operation is completed, the CPU 40 brings the imagereading apparatus 10 into the READY state of the image reading operation(step S12). In this state, the user specified the image reading modefrom the operation section 60 and presses the start key for specifyingthe start of image reading. At this time, the user chooses whether thedocument image should be read in color (color reading mode) or inmonochrome (monochromatic reading mode). After the user specifies thestart of reading, the operation section 60 supplies not only a signal torequest the start of document image reading but also informationrepresenting the document reading mode to the CPU 40.

Receiving the reading start request from the operation section 60, theCPU 40 chooses either the color reading mode or the monochromaticreading mode, on the basis of the information representing the readingmode from the operation section 60 (step S13).

When the color reading mode is chosen, the CPU 40 sets acolor-reading-mode operation according to the reading mode specified bythe user, such as a reading magnification (step S14). In the colorreading mode operation setting, for example, the reading position of theshading correction plate 11 and the reading position of the documentimage are set for the three line sensors R, G, B.

After the color reading mode operation setting is completed, the CPU 40starts to move the first carriage 18. Then, the first carriage 18accelerates to the travel speed for reading set in the operationsetting. At the reading speed, the first carriage 18 moves in the feeddirection under the shading correction plate 11 and document table 12.When the first carriage 18 has arrived at under the shading correctionplate 11, the CPU 40 reads the shading correction plate 11 with thethree line sensors R, G, B acting as the color line sensors (step S15).The reading of the shading correction plate 11 by the color line sensorswill be explained in detail later.

When the first carriage 18 passes under the shading correction plate 11and arrives at the document image reading position, the CPU 40 starts toread the image of the document placed on the document table 12 by meansof the three line sensors R, G, B acting as the color line sensors.Moreover, when the first carriage 18 has reached the reading endposition of the document, the CPU 40 ends the reading of the documentimage (step S16).

After the document image has been read, the CPU 40 moves the firstcarriage 18 to a specific standby position. When the first carriage 18has moved to the standby position, the CPU 40 returns control to stepS12 and brings the image reading apparatus 10 into the READY state.

When the monochromatic reading mode is chosen in step S13, the CPU 40sets a monochromatic-reading-mode operation according to the readingmode specified by the user, such as a reading magnification (step S17).In the monochromatic reading mode operation setting, for example, thereading position of the shading correction plate 11 and the readingposition of the document image are set for the monochromatic line sensor(black-and-white line sensor) BW.

After the monochromatic reading mode operation setting is completed, theCPU 40 starts to move the first carriage 18. Then, the first carriage 18accelerates to the travel speed for reading set in the operationsetting. At the reading speed, the first carriage 18 moves in the feeddirection under the shading correction plate 11 and document table 12.When the first carriage 18 has arrived at under the shading correctionplate 11, the CPU 40 reads the shading correction plate 11 with theblack-and-white line sensor BW acting as the monochromatic line sensor(step S18). The reading of the shading correction plate 11 by themonochromatic line sensor will be explained in detail later.

When the first carriage 18 passes under the shading correction plate 11and arrives at the document image reading position, the CPU 40 starts toread the image of the document placed on the document table 12 by meansof the black-and-white line sensor BW acting as the monochromatic linesensor. Moreover, when the first carriage 18 has reached the reading endposition of the document, the CPU 40 ends the reading of the documentimage (step S19). After the document image has been read, the CPU 40moves the first carriage 18 to a specific standby position. When thefirst carriage 18 has moved to the standby position, the CPU 40 returnscontrol to step S12 and brings the image reading apparatus 10 into theREADY state.

Next, the operation of reading the shading correction plate 11 will beexplained.

In shading correction explained below, suppose each of the line sensorsR, G, B, BW of the 4-line CCD sensor 1 reads 16 lines of data as shadingcorrection data.

FIG. 5 is a diagram to help explain the reading positions in a casewhere the line sensors R, G, B, BW read the shading correction plate 11in turn.

In the example of FIG. 5, it is assumed that the scanning positions ofthe line sensors R, G, B, BW are arranged in the order of BW, B, G, R inthe direction in which the carriage 18 moves (or in the feed direction)and that the control unit 32 takes in the signals from the 4-line CCDsensor in this order: the R signal, G signal, B signal, BW signal.

Specifically, in the example of FIG. 5, first, the red line sensor Rreads the shading correction plate 11 by 16 lines. Then, the green linesensor G reads the shading correction plate 11 by 16 lines. Then, theblue line sensor B reads the shading correction plate 11 by 16 lines.Then, the black-and-white line sensor BW reads the shading correctionplate 11 by 16 lines.

In this case, for example, if one line is 0.0425 mm in width(hereinafter, explanation will be given on the assumption that one lineis 0.0425 mm wide), the width needed for the red line sensor R to readthe shading correction plate 11 by 16 lines is 0.68 mm equivalent to 16lines.

Furthermore, the width needed for the green line sensor G to red theshading correction plate 11 by 16 lines from the point where the readline sensor R read 16 lines is the sum of 0.34 mm equivalent to thespacing of 8 lines between the red line sensor R and the green linesensor G and 0.68 mm equivalent to 16 lines (the number of readinglines).

In addition, the width needed for the blue line sensor B to read theshading correction plate 11 by 16 lines from the point where the greenline sensor G read 16 lines is the sum of 0.34 mm equivalent to thespacing of 8 lines between the green line sensor G and the blue linesensor B and 0.68 mm equivalent to 16 lines (the number of readinglines).

Moreover, the width needed for the black-and-white line sensor BW toread the shading correction plate 11 by 16 lines from the point wherethe blue line sensor B read 16 lines is the sum of 0.51 mm equivalent tothe spacing of 12 lines between the blue line sensor B and theblack-and-white line sensor BW and 0.68 mm equivalent to 16 lines (thenumber of reading lines).

Therefore, in the example of FIG. 5, the width in the feed direction ofthe shading correction plate 11 has to be equal to at least the total(3.91 mm equivalent to 92 lines) of the reading widths (equivalent to16×4 lines) of the four line sensors R, G, B. BW and the total spacingbetween the four line sensors (equivalent to 8×2+12 lines). In FIG. 5,during the time from when the reading of the shading correction plate 11is started until the reading is completed, the first carriage 18requires a travel distance L0 of 2.72 mm equivalent to 64 (16×4) lines.

Hereinafter, a first to a third reading method for the shadingcorrection method 11 will be explained.

The first to third reading methods can be applied to the image readingapparatus 10 where the scanning positions of the line sensors R, G, B,BW are arranged in the order of R, G, B, BW in the direction in whichthe first carriage 18 moves (or in the feed direction).

First, the first reading method for the shading correction plate 11 willbe explained.

FIG. 6 is a diagram to help explain the first reading method for theshading correction plate 11. In the example of FIG. 6, the scanningpositions of the line sensors R, G, B, BW are arranged in the order ofR, G, B, BW in the direction in which the first carriage 18 moves (or inthe feed direction). In FIG. 6, suppose, in the color reading mode, thecontrol unit 32 reads the signals from the three line sensors R, G, B inthis order: the B signal, the G signal, and the R signal.

In FIG. 6, in the color reading mode, with the scanning position of theblue line sensor B as the reading start position (the trailing edge inthe feed direction of the shading correction plate 11), the blue linesensor B first reads the shading correction plate 11 by 16 lines. Then,the green line sensor G reads the shading correction plate 11 by 16lines. Thereafter, the red line sensor R reads the shading correctionplate 11 by 16 lines.

Specifically, in the color reading mode, the width in the feed directionof the shading correction plate 11 is equal to the total (2.72 mm) ofthe width (0.68 mm) needed for the blue line sensor B to read 16 lines,the spacing of 8 lines (0.34 mm) between the blue line sensor B and thegreen line sensor G, the width (0.68 mm) needed for the green linesensor G to read 16 lines, the spacing of 8 lines (0.34 mm) between thegreen line sensor G and the red line sensor R, and the width (0.68 mm)needed for the red line sensor R to read 16 lines. In addition, when theshading correction plate 11 is read in the color reading mode, thetravel distance L11 of the first carriage 18 is equivalent to 48 lines(16×3 lines).

In the monochromatic reading mode, with the scanning position of theblack-and-white line sensor BW as the reading start position (thetrailing edge in the feed direction of the shading correction plate 11),the black-and-white line sensor BW reads the shading correction plate 11by 16 lines. Specifically, in the monochromatic reading mode, the widthin the feed direction needed for the black-and-white line sensor BW toread the shading correction plate 11 by 16 lines is 0.68 mm equivalentto 16 lines (the number of reading lines). In addition, when the shadingcorrection plate 11 is read in the monochromatic reading mode, thetravel distance L21 of the first carriage 18 is equivalent to 16 lines.

As described above, in the first reading method, the width in the feeddirection of the shading correction plate 11 has to be equal to thetotal (equivalent to 64 lines) of the reading widths (equivalent to 16×3lines) of at least the three line sensors R, G, and B and the totalspacing (equivalent to 8×2 lines) between the three line sensors in thecolor reading mode. In the monochromatic reading mode, the width has tobe the reading width (equivalent to 16 lines) of at least theblack-and-white line sensor BW.

Furthermore, in the first embodiment, the reading start position of theshading correction plate 11 in the monochromatic reading mode and thereading start position of the shading correction plate 11 in the colorreading mode are set separately.

Therefore, in the first reading method of FIG. 6, the shading correctionplate 11 has only to have a width of 64 lines in the feed direction.Specifically, as compared with the reading method of FIG. 5, in thefirst method, the width in the feed direction of the shading correctionplate 11 can be decreased by the total of the spacing (equivalent to 12lines) between the color line sensors (three line sensors R, G, B) andthe monochromatic line sensor (black-and-white line sensor BW) and thereading width (equivalent to 16 lines) of the black-and-white linesensor BW. In addition, the travel distance of the first carriage inreading the shading correction plate can be decreased by the traveldistance (equivalent to 16 lines) needed to read the black-and-whiteline sensor BW. This is the effect obtained because the reading position(the reading start position) of the shading correction plate in thecolor reading mode and the reading position (the reading start position)of the shading correction plate in the monochromatic reading mode havebeen set in separate positions.

Next, the second reading method for the shading correction plate 11 willbe explained.

FIG. 7 is a diagram to help explain the second reading method for theshading correction plate 11. In the example of FIG. 7, the scanningpositions of the line sensors R, G, B, BW are arranged in the order ofR, G, B, BW in the direction in which the first carriage 18 moves (or inthe feed direction). In FIG. 7, suppose, in the color reading mode, thecontrol unit 32 reads the signals from the three line sensors R, G, B inthis order: the R signal, the G signal, and the B signal.

In FIG. 7, in the color reading mode, with the scanning position of thered line sensor R as the reading start position (the trailing edge inthe feed direction of the shading correction plate 11), the red linesensor R first reads the shading correction plate 11 by 16 lines. Then,the green line sensor G reads the shading correction plate 11 by 16lines. Thereafter, the blue line sensor B reads the shading correctionplate 11 by 16 lines.

Specifically, in the color reading mode, the width in the feed directionof the shading correction plate 11 is equal to the total (1.36 mm) ofthe width (0.38 mm) obtained by subtracting the spacing of 8 lines (0.34mm) between the red line sensor R and the green line sensor G from thewidth (0.68 mm) needed for the red line sensor R to read 16 lines, thewidth (0.38 mm) obtained by subtracting the spacing of 8 lines (0.34 mm)between the green line sensor G and the blue line sensor B from thewidth (0.68 mm) needed for the green line sensor G to read 16 lines, andthe width (0.68 mm) needed for the blue line sensor B to read 16 lines.In addition, when the shading correction plate 11 is read in the colorreading mode, the travel distance L12 of the first carriage 18 isequivalent to 48 lines (16×3 lines) as is the travel distance L11 in thefirst reading method.

In the monochromatic reading mode, with the scanning position of theblack-and-white line sensor BW as the reading start position (thetrailing edge in the feed direction of the shading correction plate 11),the black-and-white line sensor BW reads the shading correction plate 11by 16 lines. Specifically, in the monochromatic reading mode, the widthin the feed direction needed for the black-and-white line sensor BW toread the shading correction plate 11 by 16 lines is 0.68 mm equivalentto 16 lines (the number of reading lines). In addition, when the shadingcorrection plate 11 is read in the monochromatic reading mode, thetravel distance L22 of the first carriage 18 is equivalent to 16 linesas is the travel distance L21 in the first reading method.

As described above, in the second embodiment, the reading start positionof the shading correction plate 11 in the color reading mode and thereading start position of the shading correction plate 11 in themonochromatic reading mode are set separately. The width in the feeddirection of the shading correction plate 11 has to be equal to thevalue (equivalent to 32 lines) obtained by subtracting the total spacingbetween the three line sensors (equivalent to 8×2 lines) from thereading widths of at least the three line sensors R, G, and B(equivalent to 16×3 lines) in the color reading mode. In themonochromatic reading mode, the width has to be the reading width(equivalent to 16 lines) of at least the black-and-white line sensor BW.

Therefore, in the second reading method of FIG. 7, the shadingcorrection plate has only to have a width of 32 lines in the feeddirection. Specifically, in the second reading method of FIG. 7, thetravel distance of the first carriage in reading the shading correctionplate is the same as in the first reading method of FIG. 6 and the widthin the feed direction of the shading correction plate is decreased bytwice the total spacing between the three line sensors.

Next, the third reading method for the shading correction plate 11 willbe explained.

FIG. 8 is a diagram to help explain the third reading method for theshading correction plate 11. In the example of FIG. 8, the scanningpositions of the line sensors R, G, B, BW are arranged in the order ofR, G, B, BW in the direction in which the first carriage 18 moves (or inthe feed direction). In FIG. 8, suppose, in the color reading mode, thecontrol unit 32 takes in the signals (the R signal, G signal, and Bsignal) from the three line sensors R, G, B at the same time.

In FIG. 8, in the color reading mode, with the scanning position of theblue line sensor B as the reading start position (the trailing edge inthe feed direction of the shading correction plate 11), the red linesensor R, green line sensor G, and blue line sensor B each read theshading correction plate 11 by 16 lines at the same time.

Specifically, in the color reading mode, the width in the feed directionof the shading correction plate 11 is equal to the total (1.36 mm) ofthe width (0.68 mm) required for each of the red line sensor R, greenline sensor G, and blue line sensor B to read 16 lines, the spacing(0.34 mm) of 8 lines between the red line sensor R and the green linesensor G, and the spacing (0.34 mm) of 8 lines between the green linesensor G and the blue line sensor B. In addition, when the shadingcorrection plate 11 is read in the color reading mode, the traveldistance L13 of the first carriage 18 is equivalent to 16 lines.

In the monochromatic reading mode, with the scanning position of theblack-and-white line sensor BW as the reading start position (thetrailing edge in the feed direction of the shading correction plate 11),the black-and-white line sensor BW reads the shading correction plate 11by 16 lines. Specifically, in the monochromatic reading mode, the widthin the feed direction needed for the black-and-white line sensor BW toread the shading correction plate 11 by 16 lines is 0.68 mm equivalentto 16 lines (the number of reading lines). In addition, when the shadingcorrection plate 11 is read in the monochromatic reading mode, thetravel distance L23 of the first carriage 18 is equivalent to 16 lines.

As described above, in the third reading method, the reading startposition of the shading correction plate 11 in the color reading modeand the reading start position of the shading correction plate 11 in themonochromatic reading mode are set separately. The width in the feeddirection of the shading correction plate 11 has to be equal to thetotal (equivalent to 32 lines) of the reading widths (equivalent to 16lines) of the line sensors R, G, B and the total spacing between thethree line sensors (equivalent to 8×2 lines) in the color reading mode.In the monochromatic reading mode, the width has to be the reading width(equivalent to 16 lines) of at least the black-and-white line sensor BW.

Therefore, in the third reading method of FIG. 8, the shading correctionplate has only to have a width of 32 lines in the feed direction.Specifically, in the third reading method of FIG. 8, the width in thefeed direction of the shading correction plate is the same as in thesecond reading method of FIG. 7, but the travel distance of the firstcarriage in reading the shading correction plate can be made smaller. Ifthe travel speed of the first carriage in the third reading method isthe same as that in the second reading method, the time required to readthe shading correction plate can be made shorter in the third readingmethod than in the second reading method. Therefore, the time needed tomake a shading correction can be made shorter.

Hereinafter, a fourth to a sixth reading method for the shadingcorrection method 11 will be explained.

The fourth to sixth reading methods can be applied to the image readingapparatus 10 where the scanning positions of the line sensors R, G, B,BW are arranged in the order of BW, B, G, R in the direction in whichthe first carriage 18 moves (or in the feed direction).

First, the fourth reading method for the shading correction plate 11will be explained.

FIG. 9 is a diagram to help explain the fourth reading method for theshading correction plate 11. In the example of FIG. 9, the scanningpositions of the line sensors R, G, B, BW are arranged in the order ofBW, B, G, R in the direction in which the first carriage 18 moves (or inthe feed direction). In FIG. 9, suppose, in the color reading mode, thecontrol unit 32 reads the signals from the three line sensors R, G, B inthis order: the R signal, the G signal, and the B signal.

In FIG. 9, in the color reading mode, with the scanning position of thered line sensor R as the reading start position (the trailing edge inthe feed direction of the shading correction plate 11), the red linesensor R first reads the shading correction plate 11 by 16 lines. Then,the green line sensor G reads the shading correction plate 11 by 16lines. Thereafter, the blue line sensor B reads the shading correctionplate 11 by 16 lines.

Specifically, in the color reading mode, the width in the feed directionof the shading correction plate 11 is equal to the total (2.72 mm) ofthe width (0.68 mm) needed for the red line sensor R to read 16 lines,the spacing of 8 lines (0.34 mm) between the red line sensor R and thegreen line sensor G, the width (0.68 mm) needed for the green linesensor G to read 16 lines, the spacing of 8 lines (0.34 mm) between thegreen line sensor G and the blue line sensor B, and the width (0.68 mm)needed for the blue line sensor B to read 16 lines. In addition, whenthe shading correction plate 11 is read in the color reading mode, thetravel distance L14 of the first carriage 18 is equivalent to 48 lines(16×3 lines).

In the monochromatic reading mode, with the scanning position of theblack-and-white line sensor BW as the reading start position (thetrailing edge in the feed direction of the shading correction plate 11),the black-and-white line sensor BW reads the shading correction plate 11by 16 lines. Specifically, in the monochromatic reading mode, the widthin the feed direction needed for the black-and-white line sensor BW toread the shading correction plate 11 by 16 lines is 0.68 mm equivalentto 16 lines (the number of reading lines). In addition, when the shadingcorrection plate 11 is read in the monochromatic reading mode, thetravel distance L24 of the first carriage 18 is equivalent to 16 lines.

As described above, in the fourth reading method, the reading startposition of the shading correction plate 11 in the color reading modeand the reading start position of the shading correction plate 11 in themonochromatic reading mode are set separately as in the first readingmethod. The width in the feed direction of the shading correction plate11 has to be equal to the total (equivalent to 64 lines) of the readingwidths (equivalent to 16×3 lines) of at least the three line sensors R,G, and B and the total spacing between the three line sensors(equivalent to 8×2 lines) in the color reading mode. In themonochromatic reading mode, the width has to be the reading width(equivalent to 16 lines) of at least the black-and-white line sensor BW.

Therefore, in the fourth reading method of FIG. 9, the shadingcorrection plate has only to have a width of 64 lines in the feeddirection. Specifically, as compared with the reading method of readingthe four line sensors R. G, B, BW in the order of R, G, B, BW, in thefourth reading method, the width in the feed direction of the shadingcorrection plate 11 can be decreased by the total of the spacing (12lines) between the color line sensors (three line sensors R, G, B) andthe monochromatic line sensor (black-and-white line sensor BW) and thereading width (equivalent to 16 lines) of the black-and-white linesensor BW. In addition, the travel distance of the first carriage inreading the shading correction plate can be decreased by the traveldistance (equivalent to 16 lines) needed for the black-and-white linesensor BW to read. This is the effect obtained because the readingposition (the reading start position) of the shading correction plate inthe color reading mode and the reading position (the reading startposition) of the shading correction plate in the monochromatic readingmode have been set in separate positions.

Next, the fifth reading method for the shading correction plate 11 willbe explained.

FIG. 10 is a diagram to help explain the fifth reading method for theshading correction plate 11. In the example of FIG. 10, the scanningpositions of the line sensors R, G, B, BW are arranged in the order ofBW, B, G, R in the direction in which the first carriage 18 moves (or inthe feed direction). In FIG. 10, suppose, in the color reading mode, thecontrol unit 32 reads the signals from the three line sensors R, G, B inthis order: the B signal, the G signal, and the R signal.

In FIG. 10, in the color reading mode, with the scanning position of theblue line sensor B as the reading start position (the trailing edge inthe feed direction of the shading correction plate 11), the blue linesensor B first reads the shading correction plate 11 by 16 lines. Then,the green line sensor G reads the shading correction plate 11 by 16lines. Thereafter, the red line sensor R reads the shading correctionplate 11 by 16 lines.

Specifically, in the color reading mode, the width in the feed directionof the shading correction plate 11 is equal to the total (1.36 mm) ofthe width (0.38 mm) obtained by subtracting the spacing of 8 lines (0.34mm) between the blue line sensor B and the green line sensor G from thewidth (0.68 mm) needed for the blue line sensor R to read 16 lines, thewidth (0.38 mm) obtained by subtracting the spacing of 8 lines (0.34 mm)between the green line sensor G and the red line sensor R from the width(0.68 mm) needed for the green line sensor G to read 16 lines, and thewidth (0.68 mm) needed for the red line sensor R to read 16 lines. Inaddition, when the shading correction plate 11 is read in the colorreading mode, the travel distance L15 of the first carriage 18 isequivalent to 48 lines (16×3 lines) as is the travel distance L14 in thefourth reading method.

In the monochromatic reading mode, with the scanning position of theblack-and-white line sensor BW as the reading start position (thetrailing edge in the feed direction of the shading correction plate 11),the black-and-white line sensor BW reads the shading correction plate 11by 16 lines. Specifically, in the monochromatic reading mode, the widthin the feed direction needed for the black-and-white line sensor BW toread the shading correction plate 11 by 16 lines is 0.68 mm equivalentto 16 lines (the number of reading lines). In addition, when the shadingcorrection plate 11 is read in the monochromatic reading mode, thetravel distance L25 of the first carriage 18 is equivalent to 16 linesas is the travel distance L24 in the fourth reading method.

As described above, in the fifth embodiment, the reading start positionof the shading correction plate 11 in the color reading mode and thereading start position of the shading correction plate 11 in themonochromatic reading mode are set separately. The width in the feeddirection of the shading correction plate 11 has to be equal to thevalue (equivalent to 32 lines) obtained by subtracting the total spacingbetween the three line sensors (equivalent to 8×2 lines) from thereading widths of at least the three line sensors R, G, and B(equivalent to 16×3 lines) in the color reading mode. In themonochromatic reading mode, the width has to be the reading width(equivalent to 16 lines) of at least the black-and-white line sensor BW.

Therefore, in the fifth reading method of FIG. 10, the shadingcorrection plate has only to have a width of 32 lines in the feeddirection. Specifically, in the fifth reading method of FIG. 10, thetravel distance of the first carriage in reading the shading correctionplate is the same as in the fourth reading method of FIG. 9 and thewidth in the feed direction of the shading correction plate is decreasedby twice the total spacing between the three line sensors.

Next, the sixth reading method for the shading correction plate 11 willbe explained.

FIG. 11 is a diagram to help explain the sixth reading method for theshading correction plate 11. In the example of FIG. 11, the scanningpositions of the line sensors R, G, B, BW are arranged in the order ofBW, B, G, R in the direction in which the first carriage 18 moves (or inthe feed direction). In FIG. 11, suppose, in the color reading mode, thecontrol unit 32 takes in the signals (the R signal, G signal, and Bsignal) from the three line sensors R, G, B at the same time.

In FIG. 11, in the color reading mode, with the scanning position of thered line sensor R as the reading start position (the trailing edge inthe feed direction of the shading correction plate 11), the red linesensor R, green line sensor G, and blue line sensor B each read theshading correction plate 11 by 16 lines at the same time.

Specifically, in the color reading mode, the width in the feed directionof the shading correction plate 11 is equal to the total (1.36 mm) ofthe width (0.68 mm) required for each of the red line sensor R, greenline sensor G, and blue line sensor B to read 16 lines, the spacing(0.34 mm) of 8 lines between the red line sensor R and the green linesensor G, and the spacing (0.34 mm) of 8 lines between the green linesensor G and the blue line sensor B. In addition, when the shadingcorrection plate 11 is read in the color reading mode, the traveldistance L16 of the first carriage 18 is equivalent to 16 lines.

In the monochromatic reading mode, with the scanning position of theblack-and-white line sensor BW as the reading start position (thetrailing edge in the feed direction of the shading correction plate 11),the black-and-white line sensor BW reads the shading correction plate 11by 16 lines. Specifically, in the monochromatic reading mode, the widthin the feed direction needed for the black-and-white line sensor BW toread the shading correction plate 11 by 16 lines is 0.68 mm equivalentto 16 lines (the number of reading lines). In addition, when the shadingcorrection plate 11 is read in the monochromatic reading mode, thetravel distance L26 of the first carriage 18 is equivalent to 16 lines.

As described above, in the sixth reading method, the reading startposition of the shading correction plate 11 in the color reading modeand the reading start position of the shading correction plate 11 in themonochromatic reading mode are set separately. The width in the feeddirection of the shading correction plate 11 has to be equal to thetotal (equivalent to 32 lines) of the reading widths (equivalent to 16lines) of the line sensors R, G, B and the total spacing between thethree line sensors (equivalent to 8×2 lines) in the color reading mode.In the monochromatic reading mode, the width has to be the reading width(equivalent to 16 lines) of at least the black-and-white line sensor BW.

Therefore, in the sixth reading method of FIG. 11, the shadingcorrection plate has only to have a width of 32 lines in the feeddirection. Specifically, in the sixth reading method of FIG. 11, thewidth in the feed direction of the shading correction plate is the sameas in the fifth reading method of FIG. 10, but the travel distance ofthe first carriage in reading the shading correction plate can be madesmaller. If the travel speed of the first carriage in the sixth readingmethod is the same as that in the fifth reading method, the timerequired to read the shading correction plate can be made shorter in thesixth reading method than in the fifth reading method. Therefore, thetime needed to make a shading correction can be made shorter.

Next, the difference between the first to sixth reading methods andtheir applications will be explained.

The first to sixth reading methods are selected suitably according tothe specifications of the image reading apparatus, including thearrangement of the scanning positions of the line sensors in the feeddirection, the order in which the signals from the sensors are taken in,and the way the signals from the sensors are taken in. This makes itpossible to design the best shading correction plate and make the mostsuitable shading correction according to various types of image readingapparatuses.

The first to third reading methods are applied to, for example, thefollowing image reading apparatuses.

The first reading method can be applied to an image reading apparatuswhere the arrangement of the scanning positions in the feed direction ofthe line sensors R, G, B, BW in the 4-line CCD sensor 1 is in the orderof R, G, B, BW and the color line sensors R, G, B are designed to readthe shading correction plate 11 in this order: B, G, R.

The second reading method can be applied to an image reading apparatuswhere the arrangement of the scanning positions in the feed direction ofthe line sensors R, G, B, BW in the 4-line CCD sensor 1 is in the orderof R, G, B, BW and the color line sensors R, G, B are designed to readthe shading correction plate 11 in this order: R, G, B.

When the arrangement of the scanning positions in the feed direction ofthe line sensors R, G, B, BW in the 4-line CCD sensor 1 is in the orderof R, G, B, BW and the order in which the color line sensors R, G, Breads can be selected, the first or second reading method can beapplied. Since the shading correction plate 11 can be made narrower inthe second reading method than in the first reading method, it isdesirable to apply the second reading method.

The third reading method can be applied to an image reading apparatuswhere the arrangement of the scanning positions in the feed direction ofthe line sensors R, G, B, BW in the 4-line CCD sensor 1 is in the orderof R, G, B, BW and the color line sensors R, G, B are designed to readthe shading correction plate 11 at the same time.

The fourth to sixth reading methods are applied to, for example, thefollowing image reading apparatuses.

The fourth reading method can be applied to an image reading apparatuswhere the arrangement of the scanning positions in the feed direction ofthe line sensors R, G, B, BW in the 4-line CCD sensor 1 is in the orderof BW, B, G, R and the color line sensors R, G, B are designed to readthe shading correction plate 11 in this order: R, G, B.

The fifth reading method can be applied to an image reading apparatuswhere the arrangement of the scanning positions in the feed direction ofthe line sensors R, G, B, BW in the 4-line CCD sensor 1 is in the orderof BW, B, G, R and the color line sensors R, G, B are designed to readthe shading correction plate 11 in this order: B, G, R.

When the arrangement of the scanning positions in the feed direction ofthe line sensors R, G, B, BW in the 4-line CCD sensor 1 is in the orderof BW, B, G, R and the order in which the color line sensors R, G, Breads can be selected, the fourth or fifth reading method can beapplied. Since the shading correction plate 11 can be made narrower inthe fifth reading method than in the fourth reading method, it isdesirable to apply the fifth reading method.

The sixth reading method can be applied to an image reading apparatuswhere the arrangement of the scanning positions in the feed direction ofthe line sensors R, G, B, BW in the 4-line CCD sensor 1 is in the orderof BW, B, G, R and the color line sensors R, G, B are designed to readthe shading correction plate 11 at the same time.

In the embodiment, as explained in the first to sixth reading method, inthe image reading apparatus having the color line sensors and themonochromatic line sensor different from the color line sensors, thereading position of the shading correction plate in the color readingmode and the reading position of the shading correction plate in themonochromatic reading mode are set independently. This makes it possibleto make narrower the width in the feed direction of the shadingcorrection plate.

From the viewpoint of carriage driving control, when the reading of theshading correction plate is started at the same position in both thecolor reading mode and the monochromatic reading mode, this makes thedriving distance longer in reading the shading correction plate. Incontrast, setting the reading start position in the color reading modeand that in the monochromatic reading mode to the optimum positionsseparately makes it possible to shorten the travel distance in readingthe shading correction plate and read the shading correction plateefficiently.

As described above in detail, with the above-described image readingmethod, the width of the reference plate can be narrower and the shadingcorrection can be made efficiently even in an image reading apparatuswhich reads images with a line sensor having color line sensors and amonochromatic line sensor. Consequently, use of the image reading methodenables the image reading apparatus to be made smaller and more stable.

Additional advantages and modifications will readily occur to thoseskilled in the art. Therefore, the invention in its broader aspects isnot limited to the specific details and representative embodiment shownand described herein. Accordingly, various modifications may be madewithout departing from the spirit or scope of the general inventiveconcept as defined by the appended claims and their equivalents.

1. An image reading apparatus which reads images on the reading face ofa document in color or in monochrome, the image reading apparatuscomprising: a line sensor which includes a plurality of color linesensors for reading a color image and a monochromatic line sensor forreading a monochromatic image different from each of the color linesensors; a scanning section on which an optical system for directinglight from the reading face of the document to the line sensor isprovided; a driving mechanism which moves the scanning section in a feeddirection with respect to the reading face of the document; a referenceplate which is provided in front of the leading edge of the reading faceof the document in the feed direction in which the scanning section ismoved by the driving mechanism; a color reference reading section which,when the reading face of the document is read in color, starts to readthe reference plate with the color line sensor at the time when scanningpositions of some of the plurality of color line sensors reach apredetermined reading start position for the reference plate; amonochromatic reference reading section which, when the reading face ofthe document is read in monochrome, starts to read the reference platewith the monochromatic line sensor at the time a scanning position ofthe monochromatic line sensor reaches a scanning start position for thereference plate set in a range from the predetermined reading startposition for the reference plate to a scanning position at which each ofthe color line sensors starts reading the reference plate by the colorreference reading section; and a correcting section which corrects theimage of the document read by each of the color line sensors, inaccordance with data obtained by reading the reference plate by thecolor reference reading section, and corrects the image of the documentread by the monochromatic line sensor in accordance with data read bythe monochromatic reference reading section.
 2. The image readingapparatus according to claim 1, wherein each color line sensor iscomposed of a plurality of line sensors whose scanning positions arearranged at specific intervals in the feed direction, and the colorreference reading section causes any one of the color line sensors tostart to read the reference plate in turn after the scanning position ofthe line sensors whose scanning position is at the rear in the colorline sensor in the feed direction reaches the predetermined readingstart position for the reference plate.
 3. The image reading apparatusaccording to claim 1, wherein each color line sensor is composed of aplurality of line sensors whose scanning positions are arranged atspecific intervals in the feed direction, and the color referencereading section causes any one of the color line sensors to start toread the reference plate in turn after the scanning position of the linesensors whose scanning position is at the head in the color line sensorin the feed direction reaches the predetermined reading start positionfor the reference plate.
 4. The image reading apparatus according toclaim 1, wherein each color line sensor is composed of a plurality ofline sensors whose scanning positions are arranged at specific intervalsin the feed direction, and the color reference reading section causesany one of the color line sensors to start to read the reference plateat the same time at the time when the scanning position of the linesensors whose scanning position is at the rear in the color line sensorin the feed direction reaches the predetermined reading start positionfor the reference plate.
 5. The image reading apparatus according toclaim 1, wherein the scanning position at which the monochromaticreference reading section causes the monochromatic line sensor to startto read the reference plate is identical with the scanning position atwhich the color reference reading section causes any one of theplurality of color line sensors to start to read the reference plate. 6.The image reading apparatus according to claim 1, wherein the pluralityof color line sensors are arranged such that the scanning positions ofthe respective color line sensors are spaced apart at specific intervalsin the feed direction, and the scanning position at which themonochromatic reference reading section causes the monochromatic linesensor to start to read the reference plate is displaced from thescanning position at which the color reference reading section causesany one of the plurality of color line sensors to start to read thereference plate, by the interval at which the color line sensors arespaced apart.
 7. The image reading apparatus according to claim 1,wherein the plurality of color line sensors are arranged such that thescanning positions of the respective color line sensors are spaced apartat specific intervals in the feed direction, and the scanning positionsof the plurality of color line sensors at which reading of the referenceplate is started are displaced at the intervals at which the color linesensors are arranged.
 8. An image reading method used in an imagereading apparatus which comprises a line sensor which includes aplurality of color line sensors for reading a color image and amonochromatic line sensor for reading a monochromatic image differentfrom each of the color line sensors, a scanning section on which anoptical system for directing light from the reading face of the documentto the line sensor is provided, a driving mechanism which moves thescanning section in a feed direction with respect to the reading face ofthe document, and a reference plate which is provided in front of theleading edge of the reading face of the document in the feed directionin which the scanning section is moved by the driving mechanism, theimage reading method comprising: when the reading face of the documentis read in color, starting to read the reference plate with the colorline sensors at the time when scanning positions of some of theplurality of color line sensors reach a predetermined reading startposition for the reference plate; when the reading face of the documentis read in monochrome, starting to read the reference plate with themonochromatic line sensor at the time a scanning position of themonochromatic line sensor reaches a scanning start position for thereference plate set in a range from the predetermined reading startposition for the reference plate to a scanning position at which each ofthe color line sensors starts reading the reference plate; andcorrecting the image of the document read by each of the color linesensors, in accordance with data obtained by reading the reference platewhen the reading face of the document is read in color, and correctingthe image of the document read by the monochromatic line sensor inaccordance with data read when reading the face of the document inmonochrome.
 9. The image reading method according to claim 8, whereineach color line sensor is composed of a plurality of line sensors whosescanning positions are arranged at specific intervals in the feeddirection, and the act of reading the reference plate with the colorline sensor is to cause any one of the color line sensors to start toread the reference plate in turn after the scanning position of the linesensors whose scanning position is at the rear in the color line sensorin the feed direction reaches the predetermined reading start positionfor the reference plate.
 10. The image reading method according to claim8, wherein each color line sensor is composed of a plurality of linesensors whose scanning positions are arranged at specific intervals inthe feed direction, and the act of reading the reference plate with thecolor line sensor is to cause any one of the color line sensors to startto read the reference plate in turn after the scanning position of theline sensors whose scanning position is at the head in the color linesensor in the feed direction reaches the predetermined reading startposition for the reference plate.
 11. The image reading method accordingto claim 8, wherein each color line sensor is composed of a plurality ofline sensors whose scanning positions are arranged at specific intervalsin the feed direction, and the act of reading the reference plate withthe color line sensor is to cause any one of the color line sensors tostart to read the reference plate at the same time at the time when thescanning position of the line sensors whose scanning position is at therear in the color line sensor in the feed direction reaches thepredetermined reading start position for the reference plate.
 12. Theimage reading method according to claim 8, wherein the scanning positionat which the monochromatic line sensor starts to read the referenceplate is identical with the scanning position at which the colorreference reading section causes any one of the plurality of color linesensors to start to read the reference plate.
 13. The image readingmethod according to claim 8, wherein the plurality of color line sensorsare arranged such that the scanning positions of the respective colorline sensors are spaced apart at specific intervals in the feeddirection, and the scanning position at which the monochromatic linesensor starts to read the reference plate is displaced from the scanningposition at which the color reference reading section causes any one ofthe plurality of color line sensors to start to read the referenceplate, by the interval at which the color line sensors are spaced apart.14. The image reading method according to claim 8, wherein the pluralityof color line sensors are arranged such that the scanning positions ofthe respective color line sensors are spaced apart at specific intervalsin the feed direction, and the scanning positions of the plurality ofcolor line sensors at which reading of the reference plate is startedare displaced at the intervals at which the color line sensors arearranged.